Overview

Brief Summary

Introduction to the Genus Inga

The Genus Inga Mill. (1754) belongs to the Subfamily Mimosoideae tribe Ingeae Benth. (1875) and comprises near 300 species of trees with a strictly Neotropical distribution. Brazil harbors the greatest number of species (near 140 spp.), followed by Peru (92 spp.), Colombia (near 76+ spp.) and Ecuador (75 spp.) (Pennington, 1997). Due to its great number of species, the Genus Inga is subdivided into 14 Taxonomic Sections characterized by shared morphological attributes (Pennington, 1997).

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Distribution

General Distribution of the Genus

Neotropical, extending into temperate zones in the North of Mexico and in the South of Uruguay. The distributional range comprises from Durango and Coahuila (aprox. 25 degrees N)in Mexico, to the delta of Rio de la Plata (aprox. 34 degrees S) in Uruguay. The genus is abundant in the pluvial montane forests and in the lowland pluvial forests along the Neotropical humid zone, and occupies habitats from sea level to 3000 m of elevation. Inga is restricted to riparian habitats and in periodically flooded areas, although many species are confined to pluvial forests in non-flooded areas. Inga species occupy humid environments with well drained soils.

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Physical Description

Look Alikes

Inga look alikes

The genus Inga is generally regarded as very easy to identify within the species of the Neotropical flora. Its characteristic generic morphological uniformity makes it completely unmistakable from other Neotropical tree genera. However, there is one legume species that resembles very closely the morphology of the genus Inga. This legume tree species belongs to the genus Cojoba (which is also a member of the subfamily Mimosoideae as Inga is). The species of Cojoba that very close resembles one of the genus Inga is Cojo rufescens (Cojoba rufescens (Benth.) Britt. & Rose). This species vegetatively resembles very close an Inga species, but although the leaves are simply pinnate and the structure of the flowers is similar, in Cojoba rufescens the legume is dehiscent, moniliform and the interior of the legume valves is deep reddish in color; on the contrary, in Inga the legume is indehiscent, the legume is continuous (not moniliform), and the valves never have red coloration.

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Development

Seed and Seed Development

The Inga seed structure is completely distinguishable from other genera of the Mimosoideae. The white, "cotton like", seed coat is produced by the Malpighian cells of the integument, which become an hypertrophied layer (Maumont, 1990). This enlarged, sugar-rich Malpighian cells form the white edible sarcotesta, which is the attractant for many dispersing animals. Germination of Inga seeds need correct conditions of moisture and shade. The seeds usually begin to germinate inside the unopened pod with the splitting of the seed coat followed by the development of a radicle of several centimeters long. The embryo grows quickly, beginning with the elongation of the radicle and followed by the growth of the cotyledons which become green and photosynthetic. The first pair of leaves often have one pair of pinnae and are opposite, the petiole appears also winged. The leaves that will form later are always arranged spirally. Leaves always have stipules (including the first-formed leaves), and their shape is usually much narrower than that shown by the adult leaves.

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Ecology

Habitat

Chocó-Darién Moist Forests Habitat

This taxon can be found in the Chocó-Darién moist forests ecoregion, one of the most species rich lowland areas on Earth, with exceptional abundance and endemism over a broad range of taxa including plants, birds, amphibians and arthropods. The biological distinctiveness is exceptional, with considerable biodiversity.

There are three principal geomorphologic types in the ecoregion: alluvial plains of recent origin, low mountains formed by the relatively recent dissection of sediments from the Tertiary and Pleistocene periods, and the complexes in mountain areas consisting of mesozoic rocks. The high precipitation and the topography mean that the ecoregion includes a complex of great hydrographic basins, the most important being those of the Atrato, Baudó, and San Juan Rivers and the Micay and Patía Rivers in the south. The force of the water in many of these rivers form deep gorges cutting through the mountains, creating spectacular rapids and waterfalls in the mountains. At lower elevations, large rivers become very wide and with many meanders. Given the high precipitation in the region, it is not surprising that the soils are severely leached and poor in nutrients. Most of the ecoregion has typical laterite soils with reddish clay, although the soils are younger and less leached in some areas, especially close to the base of the Andes and in the floodplains of the major rivers. Of particular botanical interest are the white clay soils in the region of Bajo Calima in Colombia, which are associated with the gigantic sclerophyllous leafed and unusually large fruited vegetation.

Depending on the altitudinal gradient, soil water content and the effect of the sea, there are various types of vegetation that make up the ecoregion. In broad terms, in the northern part of the ecoregion, the lowland rainforests correlate to the Brosimun utilis alliance, including communities dominated by the deciduous Cuipo tree (Cavanillesia platanifolia), the Espavé wild cashew (Anacardium excelsum), the Panamanian rubber tree (Castilla elastica), Brosimum guianense, Bombacopsis spp., Ceiba pentandra, Dipteryx panamensis, and others. In the undergrowth Mabea occidentalis, Clidemia spp., Conostegia spp. and Miconia spp. are abundant. In zones that are occasionally flooded, the Cativo (Prioria copaifera) flourishes as well. In the southern part of the ecoregion, these rainforests have multiple strata, with two layers of trees, lianas, and epiphytes with vigorous growth rates. The number of deciduous plants increases in the north and south, where there is a dry season, particularly near the coast. The forests at higher altitudes, starting at 600 meters, have communities with the following species: Guamos (Inga spp.), Billia columbiana, Brosimum sp., Sorocea spp., Jacaranda hesperia, Pourouma chocoana, Guatteria ferruginea, Cecropia spp., Elaegia utilis, and Brunellia spp.

There are 577 species of birds recorded; Tyrannidae is listed as the most diverse avian family, presenting 28 genera and 60 species within the ecoregion. The Choco-Daroemis is a center of avian endemism of the Neotropics; moreover, according to Stattersfield, this ecoregion spans two Endemic Bird Areas, one in Central America and one in South America.

Between these two Endemic Bird Areas there are over sixty restricted range species, including the Chocó tinamou (Crypturellus kerriae VU), Chestnut-mantled Oropendola (Psarocolius cassini EN), Viridian dacnis (Dacnis viguieri), Crested ant-tanager (Habia cristata), Lita woodpecker (Piculus litea), and Plumbeous forest-falcon (Micrastur plumbeus EN). Also to be noted is the presence of the Harpy eagle (Harpia harpyja), the Black and white crowned eagle (Spizastur melanoleucus), taxa increasingly rare in many areas of the Neotropics, and possibly the Speckled antshrike (Xenornis setifrons EN) although one has not been recorded in Colombia since the 1940s.

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General Habitats

The genus Inga occupies mainly wet forest habitats. For successful germination the seeds of Inga require shade and high humidity. A few number of known species can occupy semi-arid areas, although they are restricted to temporary or permanent water courses. Within the wet-rainy forests, members of Inga can be found in a variety of niches. They can represent understorey elements as well as properly canopy trees. Many of the known species require high light conditions for development and can be found as a gregarious species. Many others are typically found in secondary forest conditions with high rates of growth and with really large leaves that allow them to success over other competitors. These species in particular prefer poorly drained or periodically inundated areas, swamps, and riverbanks.

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Dispersal

Dispersal in Inga

The seed coat, the white edible sarcotesta with cotton-like appearance, constitutes the attractant for the dispersing animals (mostly primates). In order to reach to the sweet sarcotesta, dispersers and predators can eat from one of the sides of the pod, thus taking the seeds individually, or they eventually peel the pod from the end tip like a banana. Besides primates, which constitute the main known dispersers of Inga, other animals that could potentially act as a dispersor agents are some species of parrots.

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General Ecology

Inga and insects

The genus Inga is known by neotropical farmers to attract a high diversity and abundance of insects to its vicinities. Grasshoppers, katydids, and larvae of Lepidoptera are among the insects feeding on Inga leaves for food (Koptur, 1983b). Leaf cutter ants also use the leaves of Inga but as a substrate on which to cultivate the fungus that constitutes the sole food for their larvae (Nichols-Orianz and Schultz, 1990). The fruit fly Anastrepha distincta (Diptera: Tephritidae) infest a small percentage of the pods of some Inga species (Hernandez-Ortiz and Perez-Alonso, 1993); beetle larvae also commonly exploit the fruits of Inga (e.g. the genus Conotrachelus, Coleoptera:Curculionidae: Molytinae; also genera from the families Nitidulidae and Staphylinidae) (Valente and Gorayeb, 1984). Small wood-boring insects (Platypus ratzeburgi) are found to attack species of Inga (Gallardo, 1987).

Different insect taxa represent floral visitors and pollinators for Inga, including 19 families representing 5 orders: Hemiptera, Coleoptera, Diptera, Hymenoptera, and Lepidoptera (Koptur, 1983a). Effective pollinators in Inga are represented by hummingbirds and species of the lepidopteran families Hesperiidae, Sphingidae, and Uranidae.

Up to one third or more of the developing foliage of species of Inga may be eaten by herbivores (Kursar et al. 2009). It has been shown that as high as up to 43 species of Inga may coexist at a single site, and this may in part be explained because species differ considerably in antiherbivore defences, the defences varying independently between the species (Kursar et al. 2009).

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Nodulation and Nitrogen Fixation in Inga

Various authors have reported nodulation in several species of Inga (Allen & Allen, 1981; Moreira et al., 1992) as well as effective associations between different Inga species and mycorrhizal fungi. Rhizobial partners have also been found in members of Inga (Allen & Allen, 1939; de Faria, 1995). The mycorrhizal fungi that have been found to be associated with Inga belong to the group endomycorrhizae (Reategui et al., 1995). In many species of Inga nodulation has been reported, but N2-fixation has only been quantified in few of these species (Roskoski and Van Kessel, 1985).

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Life History and Behavior

Growth

Growth and Habit in Inga

Inga members range from treelets and trees up to 40 m in height, and usually presenting a smooth cylindrical trunk. Most of the species have smooth bark with numerous lenticels all over it, these structures can be arranged in either horizontal or vertical rows on the stem. Buttresses are found in few species which get large dimensions.

Throughout the genus, leaf arrangement and branching pattern appear constant. This pattern agrees to the architectural model of Troll (Halle and Oldeman, 1970), in which all axes are plagiotropic from an early stage. The broad, shallow, horizontal crown typical of Inga members is produced by the distichous phyllotaxis and horizontal growth of the branches. Poncy (1985) discusses with great detail the growth of the young Inga plants.

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Evolution and Systematics

Evolution

Evolution of the Genus Inga

Inga, with near 300+ species constitutes an important component of Neotropical lowland forests. DNA sequence data is consistent with a recent diversification in the Genus (molecular evidence suggests that Inga seems to have diversified within the last two million years, Richardson et al. 2001). Richardson and colleagues (2001) estimated that speciation in Inga had to be concentrated in the past 10 million years, with the bulk of the species arising in the last 2 million years. This chapter of extensive diversification coincides (according to the authors) with relatively recent major geological phenomena, such as uplift of the Andes, the bridging of the Isthmus of Panama, and Quaternary glacial fluctuations.

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Conservation

Conservation Status

Conservation Status of Inga

The Conservation Status of Inga has not been evaluated thoroughly. The IUCN inventory of the global conservation status of species for the Genus Inga has not been addressed yet. Since the status of the some of the widely cultivated species (e.g., Inga edulis) is probably well known, the majority of the species remain poorly known or undercollected and their conservation status remain uncertain for science.